Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, lithium secondary batteries having a high-energy density and voltage, a long cycle lifespan and a low self-discharge rate are commercially available and widely used.
As a cathode active material for the lithium secondary battery, lithium-containing cobalt oxide (LiCoO2) is largely used. In addition, consideration has also been made of using lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxides (LiNiO2).
Of the aforementioned cathode active materials, LiCoO2 is currently widely used due to superior general properties such as excellent cycle characteristics, but suffers from disadvantageous problems such as low safety, expensiveness due to finite resources of cobalt as a raw material, and the like. Lithium manganese oxides such as LiMnO2 and LiMn2O4 are abundant resource materials and advantageously employ environmentally-friendly manganese, and therefore have attracted a great deal of attention as a cathode active material capable of substituting LiCoO2. However, these lithium manganese oxides suffer from shortcomings such as a low capacity and poor cycle characteristics.
Whereas, lithium/nickel-based oxides such as LiNiO2 are inexpensive as compared to the cobalt-based oxides and exhibit a high discharge capacity upon charging to 4.25 V. The reversible capacity of doped LiNiO2 approximates about 200 mAh/g which exceeds the capacity of LiCoO2 (about 153 mAh/g). Therefore, despite somewhat lower average discharge voltage and volumetric density of LiNiO2, commercial batteries containing LiNiO2 as a cathode active material exhibit an improved energy density. To this end, a great deal of intensive research is being actively undertaken on the feasibility of applications of such nickel-based cathode active materials for the development of high-capacity batteries. However, the LiNiO2-based cathode active materials still suffer from some weakness which have not been sufficiently solved, such as high production costs, swelling due to gas evolution in the fabricated batteries, poor chemical stability, high pH and the like.
Many prior arts focus on improving properties of LiNiO2-based cathode active materials and manufacturing processes of LiNiO2. For example, a lithium-transition metal composite oxide has been proposed wherein a portion of nickel is substituted with another transition metal element such as Co, Mn, etc.
Lithium transition metal active materials containing two or more materials of Ni, Co and Mn cannot be easily synthesized by a simple solid-state reaction. There is known in the art a technique using a transition metal precursor prepared by co-precipitation or the like, as a precursor for the preparation of such lithium transition metal active materials.
This type of transition metal precursor has been studied to prepare lithium transition metal oxides which are intended to express desired performance through the prevention of tap density lowering by control of the particle size or the optimization of particle shape by spheronization or the like.
In spite of various attempts which have been made as above, there is a strong need in the art for the development of a lithium transition metal oxide having satisfactory performance and a transition metal precursor for preparing the same.